The present invention relates to a thread cutting tool for cutting female threads, with a shaft and a cutting section that is composed of several axially parallel or spirally arranged cutter rows each provided with a group of cutting teeth and chip grooves arranged in between them.
The term “thread cutting tool” generally includes so-called screw taps, and also certain types of thread milling cutters to the extent that they are for manufacturing female threads. Such a thread cutting tool either itself produces the thread bore, i.e., the core boring for a thread, or it is screwed into a previously prepared bore, the diameter of which is usually in proportion to the final thread diameter, depending on the material of the workpiece. The thread cutting tool is screwed into the bore with a forward movement matched accurately to its speed of rotation, wherein the cutting teeth, which are arranged in several substantially parallel groups (known as cutter rows), cut the threads. In the case of a single-flight thread, the distance apart of the cutting teeth, respectively arranged in the axial direction behind one another, of a group, exactly corresponds to the thread pitch.
Generally, in the forward area of such a tool, i.e., the so-called initial cut portion, the external diameter of the cutting tool increases in the axial direction rearwards from the tip, whereby the axially forward teeth are of smaller radii. Such axially forward cutting teeth are configured wider, and cut the thread grooves, while the following cutting teeth are arranged on an increasingly larger radius, and in this way cut out the thread grooves lying between the thread flanks to the desired depth. With increasing tooth diameter, that is to say the distance from the base of the groove, the thread grooves become increasingly narrow, and the cutting teeth are also correspondingly configured with a narrower land. The cross-section of the cutting teeth corresponds to the cross-section of the thread grooves to be produced, so the auxiliary cutting edges of the cutting teeth come together at the flank angle of the thread, while the main cutting edge cuts the base of the thread groove. Behind the initial cut portion there is located the guiding area where although the cutter rows still have teeth, these do not cut or hardly cut, and instead simply provide continuous precise guidance of the thread cutting tool.
The forward cutting teeth in particular, that is to say the cutting teeth in the initial cut portion area, which do not cut the thread groove to full depth, are configured correspondingly wide, as they create the further portion of the thread profile. The flank adjoining the main cutting edge of a cutter row generally has a clearance angle in the order of 0.5° to 10°, typically of 1° to 2° relative to the circumferential surface created by the respective main cutting edge.
Once an appropriate thread cutting tool has been screwed into an appropriate bore to the desired depth, and has thus cut the desired thread, the tool is axially withdrawn. During the axial withdrawing movement the tool must be rotated in the opposite direction, matching the thread pitch, and is thus effectively unscrewed from the thread that has been produced. In cutting the thread, however, chippings are also inevitably produced from the material of the workpiece, which, in particular in the case of blind bores, are only partially conveyed out of the thread bore during thread cutting. Thus, when the thread cutting tool is unscrewed from the thread bore, some chippings may stick between the cutting teeth and the thread that has been produced, and not only damage the thread that has been produced but also contribute to a larger degree of wear and tear on the thread rows. In particular, in the case of very hard materials such as, for example, highly alloyed steels, the chippings that become stuck to the flanks of the cutting teeth lead to portions of the cutting edges breaking off. In particular, in the case of thread cutting tools wherein at least the cutting teeth are composed of solid carbide (to be used for cutting in particularly hard substances), the danger of breakage of cutting edges because of adhering chippings is particularly great. In general, adhering chippings lead to faster wear and tear of thread cutting tools.
Given this background, the object of the present invention is to provide a thread cutting tool with the features described in the introduction, wherein wear and tear is considerably reduced and which has significantly greater durability than conventional thread cutting tools. These advantages will come to the fore particularly when cutting threads in relatively hard and/or ductile metallic substances such as, for example, fine steel. In particular, by means of the present invention the cutting edges of the cutting teeth will suffer less damage and in particular less breakages even after prolonged use of the thread cutting tool.